Cytomolecular characterization of de novo formed rye B chromosome variants

Abstract

Background: B chromosomes (Bs) are dispensable elements which occur in many species including rye (Secale cereale). We determined the organization of B variants to obtain insights into the origin of B polymorphisms in rye.

Results: The observed B variants were classified according to their morphology and in situ hybridization patterns with the B-specific repeats D1100 and CL11 into (I) long arm iso B, (II) D1100-deficient B and (III) small metacentric B variants. Long arm iso Bs are likely products of a meiotic centromere misdivision and subsequent duplication of the long arm, whereas small B variants are probably generated by chromosome breakage. Some deficient Bs experienced extensive amplification of CL11 repeats.

Conclusions: Both the pericentromere and the nondisjunction control region seem to be involved in the generation of rye B chromosome variants. However, due to the loss of the B-specific nondisjuction control region most of the variants generated are not capable to accumulate in a population.

Figure 1

Characterization of mitotic rye (Secale cereale) B chromosome variants by FISH with the B-specific probes D1100 and CL11, the microsatellite (CAA)₁₀, as well as with the Arabidopsis-type telomere repeat. (a) Standard Bs and its structural variants found in the rye line 7415. The B variants are classified into three different subtypes. Bar equals 10 μm. (b) Plant #11 carrying a long arm iso B, (c) Plant #4 carrying a small deficient B lacking the long arm, (d) Plant #6 carrying four standard Bs and one small B variant (arrow). (e) Standard B showing telomeric signals on both arms and a pericentromeric CL11 signal, (f) B-variant almost entirely labeled with CL11 sequence but with telomeric signals on one chromosome arm only, (g) B variant showing a high accumulation of CL11 signals and telomeric signals only on the short arm, (h) two small B variants, (i) metaphase of a wheat plant with rye short arm iso B showing telomeric signals on both arms of a small iso B. Inserts highlight B variants. Bar equals 20 μm.